1. Surgical Diathermy and the Indifference Plate
Surgical diathermy is an electrical method of producing cuts in human tissue. This is done by striking an electric arc by a stylus which completes an electric circuit through the human body. A typical frequency of operation is 600 kHz and the range of power outputs from a diathermy set varies from a few Watts to about 400 W. A diagrammatic view of a typical arrangement is shown in FIG. 1, where:
Z1 represents the contact impedance between the active electrode and the patient, say 400 ohm, mainly resistive;
Z2 represents the impedance of the current path within the patient's body, typically 20 ohm, mainly resistive;
Z3 represents the contact impedance between the plate electrode and the patient, typically 2 ohm, mainly resistive; and
Z4 represents the impedance of the plate cable, typically 30 ohm, mainly inductive.
The return path of the electric current is conveyed from the human body to the diathermy machine by a metal plate of sufficient area that the current density is harmlessly low. This is called the "neutral plate", "indifference plate" or simply "plate electrode".
The indifference plate has undergone some development. Initial types were of lead sheet bound to the patient by dressings and with saline solution added to give good electrical conductivity. Modern types are of foil, the more recent types having an adhesive conductive film which gives good electrical contact with the patient and high security of fixing. The claimed contact impedance of one example of this adhesive type is 15 ohms.
2. The Capacitive Type of Indifference Plate
An alternative approach, which also claims an improved current distribution over former types is the capacitive type. This also is a foil type with an adhesive layer. This adhesive, however, is not conductive but an insulator, so that the return circuit is made through a capacitive reactance. A measured value of the reactance of a commercial plate of this type when attached to a stiff aluminium plate was found to be around 30 ohms. This is twice the contact impedance of the above-mentioned conductive plate and 15 times that of a typical plate electrode of FIG. 1. Some crude measurements of the impedance of a capacitive type plate when on human flesh yielded a value of about 80 ohms.
The advantage of this type of plate is that, when the plate is pulled from the skin, as might accidently but rarely happen, then the reactive impedance is so sized that as the plate area diminishes, the impedance rises quickly to a value that limits the total current and hence the total power and power density delivered. This effect is shown by the curves in FIGS. 2 and 3 where both the total power and the power density (i.e. power dissipated per unit area of skin) are held to moderate levels in the capacitive type of neutral plate for the maximum power settings on a diathermy machine. Although in this particular case `safe` limits as shown arbitrarily in these graphs are exceeded, it is evident that the possibility of a severe burn is very much reduced. Indeed a redesign, by for instance thickening the dielectric of the neutral plate, would easily permit `safe` limits to be held.
Note that a power density of 300 mW/sq cm is about the value at the surface of a 25 cm 60 W incandescent strip light and appears quite bearable to the touch, whereas the power density of 600 mW/sq cm at the surface of a 60 W pearl lamp is distinctly unbearable. Of course there are several complicating factors in the above picture, not least being that the surgeon applies power in an intermittent fashion, which would permit somewhat higher short term power densities. The overall conclusion is, however, that the capacitive type of plate is a fail-safe device in the type of accident where the plate is pulled off.
The other advantage claimed for capacitive type plates is similar to the last-mentioned one, but concerns normal use. In this case it is evident that the plate or adhesive does not actually have to touch the skin to achieve a capacitive value. A much more even distribution of current is therefore likely than from the conductive type of plate, which relies on special preparation to achieve good plate skin contact resistance. Even with the modern self-adhesive conductive types it is possible that current density is higher at the edges than directly under the plate where it might not be so firmly pressed down. In short, hot spots are inherently avoided.
The capacitive type neutral plate has, however, the following disadvantages:
a. Because of the somewhat higher impedance of the capacitive plate as presently manufactured, it does mean that for earth-referenced diathermy machines the patient (FIG. 4) can be at a significant potential, Vf, with reference to earth and any inadvertent contact of the patient to earthed metal could allow current to flow and an RF burn to be sustained (FIG. 4A). Because of the low contact resistance, Z3, of the resistive neutral plate, this accident is much less likely to produce a damaging current. For FIG. 4:
Z5 represents impedance of alternative internal current path;
Z6 represents contact impedance of patient to metal object;
Z7 represents impedance to earth of metal object; and
Z8 represents the overall impedance (mainly capacitive) of the patient to earth.
b. Again because of the high impedance of the capacitive neutral plate any shunt path can carry a current of damaging proportions. Thus if there is a point failure in the insulation, or if an uninsulated tab were to contact the skin then a large proportion of the current could be carried through this failure at very high power density. This path is represented by any impedance in parallel with Z3.
The possibility of this type of event occurring has led to this type of neutral plate not being used except for applications below 50 W.